docs: add outlookidfix config option to sendemail documentation

docs: add outlookidfix config option to sendemail documentation

The documentation for command line option `--outlook-id-fix` is there in
the sendemail documentation, but the config option `sendemail.outlookidfix`
was missing. Add the same to the documentation.

White at it, also enclose the values `true` and `false` in backticks in
the documentation for `sendemail.mailmap`.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>